1. Field of the Invention
This invention relates to a type of semiconductor laser which is capable of operating either at a long wavelength or at a number of different wavelengths distributed over a broad range, this result being achieved by making use of monocrystalline layers made of different materials which are all compatible with the crystal lattice of a single compound designated as III-V (namely a compound formed of elements of the third and fifth columns of the Periodic Table). This type of laser is applicable in particular to the field of optical fiber telecommunications.
2. Description of the Prior Art
Optical fibers employed in optical telecommunications have a minimum absorption for wavelengths in the vicinity of 1.3 and 1.6 .mu.m. It is therefore an advantage to use emitters which operate within this range.
Conventional semiconductor lasers utilize heterostructures of semiconductor materials which belong:
either to the ternary system: Ga, Al, As; PA1 or to the quaternary system: Ga, In, As, P. PA1 a predetermined monocrystalline substrate; PA1 a sandwich comprising three monocrystalline layers which have the same crystal lattice parameter as the substrate, form at least one semiconductor junction between each other, and also satisfy the following two conditions:
The materials of the second system alone permit laser emission within the minimum absorption range of optical fibers employed in telecommunications.
The aim of the invention is to meet industrial requirements in the field of optical fibers by providing heterostructure semiconductor lasers having a structure which is different from the conventional heterostructures mentioned in the foregoing and offers certain advantages of an industrial character.
It is recalled that the heterostructure of a semiconductor laser is constituted as follows:
(1) The central layer is formed of a semiconductor chosen so as to ensure that the forbidden band or energy gap corresponds to a predetermined wavelength; this layer forms the active zone; PA2 (2) The outer layers have energy gaps of greater width than that of the central layer in order to create a confinement of the electron carriers in the central layer, hence the name of confinement layers given to these outer layers.
Known forms of construction thus include lasers in which provision is made for an active layer of GaAs on a GaAs substrate and confinement layers of Al.sub.x Ga.sub.1-x As, or lasers in which provision is made for an active layer of Ga.sub.x In.sub.1-x As or Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y and confinement layers of InP on an InP substrate.